1. Field of the Invention
The invention relates to a method for improving access to a newly defined logical unit in a storage system and in particular relates to methods and structures for creating a RAID logical unit so as to make it available immediately for processing of host I/O requests.
2. Description of Related Art
Typical computing systems store data within devices such as hard disk drives, floppy drives, tape, compact disk, etc. These devices are otherwise known as storage devices. The storage capacity of these storage devices has rapidly increased as computing applications"" demand for storage have increased. Simultaneous with the increase in capacity, computer applications and users have demanded increased performance. Computing applications have become highly dependent on high performance, high capacity storage devices. However, such increased dependency on storage devices underscores the need for increased reliability of such storage subsystems. Failure of such high capacity, high performance storage devices and subsystems can cripple vital computing applications.
Disk array storage systems provide both improved capacity and performance as compared to single disk devices. In a disk array storage system, a plurality of disk drives are used in a cooperative manner such that multiple disk drives are performing, in parallel, the tasks normally performed by a single disk drive. Striping techniques are often used to spread large amounts of information over a plurality of disk drives in a disk array storage system. So spreading the data over multiple disk drives improves perceived performance of the storage system in that a large I/O operation is processed by multiple disk drives in parallel rather than being queued awaiting processing by a single disk drive.
However, adding multiple disk drives to a storage system reduces to reliability of the overall storage system. In particular, spreading data over multiple disk drives in a disk array increases the potential for system failure. Failure of any of the multiple disk drives translates to failure of the storage system because the data stored thereon cannot be correctly retrieved.
RAID techniques are commonly used to improve reliability in disk array storage systems. RAID techniques generally configure multiple disk drives in a disk array in geometries that permit redundancy of stored data to assure data integrity in case of various failures. In many such redundant subsystems, recovery from many common failures can be automated within the storage subsystem itself due to the use of data redundancy, error codes, and so-called xe2x80x9chot sparesxe2x80x9d (extra disk drives that may be activated to replace a failed, previously active disk drive). The 1987 publication by David A. Patterson, et al., from University of California at Berkeley entitled A Case for Redundant Arrays of Inexpensive Disks(RAID), reviews the fundamental concepts of RAID technology.
RAID level zero, also commonly referred to as disk striping, distributes data As stored on a storage subsystem across a plurality of disk drives to permit parallel operation of a plurality of disk drives thereby improving the performance of I/O write requests to the storage subsystem. Though RAID level zero functionality improves I/O write operation performance, reliability of the disk array subsystem is decreased as compared to that of a single large disk drive. To improve reliability of disk arrays, other RAID geometries for data storage include generation and storage of redundancy information to permit continued operation of the disk array through certain common failure modes of the disk drives in the disk array.
There are six other xe2x80x9clevelsxe2x80x9d of standard RAID geometries that include redundancy information as defined in the Patterson publication. Other RAID geometries have been more recently adopted and utilize similar concepts. For example, RAID level six provides additional redundancy to enable continued operation even in the case of failure of two disk drives in a disk array.
The simplest array, a RAID level 1 system, comprises one or more disks for storing data and an equal number of additional xe2x80x9cmirrorxe2x80x9d disks for storing copies of the information written to the data disks. The remaining RAID levels, identified as RAID levels 2, 3, 4 and 5 systems by Patterson, segment the data into portions for storage across several data disks. One or more additional disks are utilized to store error check or parity information. RAID level 6 further enhances reliability by adding additional redundancy information to permit continued operation through multiple disk failures. The methods of the present invention may be useful in conjunction with any of the standard RAID levels.
RAID storage subsystems typically utilize a control module (controller) that shields the user or host system from the details of managing the redundant array. The controller makes the subsystem appear to the host computer as one (or more), highly reliable, high capacity disk drive. In fact, the RAID controller may distribute the host computer system supplied data across a plurality of the small independent drives with redundancy and error checking information so as to improve subsystem reliability. The mapping of a logical location of the host supplied data to a physical location on the array of disk drives is performed by the controller in a manner that is transparent to the host system. RAID level 0 striping for example is transparent to the host system. The data is simply distributed by the controller over a plurality of disks in the disk array to improve overall system performance.
RAID storage systems generally subdivide the disk array storage capacity into distinct partitions referred to as logical units (LUNs). Each logical unit may be managed in accordance with a selected RAID management technique. In other words, each LUN may use a different RAID management level as required for its particular application.
A typical sequence in configuring LUNs in a RAID system involves a user (typically a system administrator) defining storage space to create a particular LUN. With the storage space so defined, a preferred RAID storage management technique is associated with the newly created LUN. The storage space of the LUN is then typically initializedxe2x80x94a process that involves formatting the storage space associated with the LUN to clear any previously stored data and involves initializing any redundancy information required by the associated RAID management level.
The formatting and other initialization a LUN is generally a time-consuming process. Formatting all storage space for a large LUN may take several minutes or even hours. Often, the initialization processing further includes a verification step to verify proper access to all storage space within the LUN. Such verification may include writing and reading of test data or commands on each disk of the LUN to verify proper operation of the LUN. Errors detected in this verification process that relate to recording media defects are then repaired if possible (i.e., by allocating a spare sector or track of the associated disk drive to replace a defective one). This verification process is yet another time consuming aspect of the LUN initialization process.
Most presently known RAID storage systems preclude processing of host I/O requests for the logical unit until after the initialization has completed. In such systems, the logical unit is said to be unavailable until initialization has completed. Some RAID storage systems have improved upon these known systems by making the logical unit available as soon configuration information has been saved to define the mapping of data within the logical unit and as soon redundancy information has been initialized for the entire LUN. Time consuming disk media verification is skipped in lieu of sensing and correcting such errors as the LUN is used.
It remains a problem however unit in such improved systems, to make the logical unit available as soon as possible for processing of host I/O requests. Initialization of redundancy information still requires significant amount of time wherein large logical unit is defined. Further, presently known logical unit initialization sequences cease operation of the LUN when one of the drives in the logical unit fails. Further access to the logical unit having failed drives is then precluded in presently known RAID systems.
It is therefore the problem to make a newly defined RAID logical unit available for host I/O processing as soon as possible, preferably even during initialization. It is a further problem to assure that the logical unit is available for processing of host I/O requests unit when a drive in the LUN is sensed to have failed during the initialization sequence.
The present invention solves the above and other problems, thereby advancing the state of the useful arts, by providing methods and associated structure to permit immediate availability of newly defined LUNs as soon as possible after configuration information regarding the LUN has been saved. The present invention allows I/O requests to be performed on a logical unit in parallel with initialization of the redundancy information in the LUN. Furthermore, the present invention assures continued availability of the logical unit and eventual completion of LUN initialization despite failure of a disk drive during the initialization process. Methods of the present invention permit such a LUN with a failed drive to continue operation though in a degraded mode.
More specifically, methods and structures of the present invention permit immediate availability of a newly created logical unit for processing host I/O requests while redundancy information for the LUN is being initialized. Methods of the present invention maintain boundary information regarding progress in initializing redundancy information for the LUN. Host I/O requests for writing of data below this boundary level (i.e., in an area of the LUN for which redundancy information has not yet been initialized) are performed in a manner that assures valid redundancy data is generated and written. Conversely, host I/O requests for writing of data above the boundary level (i.e., in an area of the LUN for which valid redundancy information has already been initialized) proceed in a normal manner in accordance with the associated RAID level for the LUN.
In a further aspect of the present invention, initialization of redundancy information in accordance with the present invention as described above is terminated in response to detection of a failed disk drive in the LUN. However, unlike prior techniques, the LUN remains available for processing of HOST I/O requests though in a degraded mode. When the failed disk drive is eventually replaced, standard RAID regeneration techniques to recover loss data are used to complete the initialization of redundancy information for the LUN.
Another aspect of the invention lies in the capability to resume the initialization process. The present status of the initialization is maintained as each portion of the LUN is initialized. As the boundary separating initialized from un-initialized areas of the LUN is moved, checkpoint data is saved to enable recovery at any point in the initialization. If the initialization is interrupted, such as due to a power loss, the checkpoint information is used to resume the initialization where it left off.
It is therefore an object of the present invention to provide methods and associated structure that permit immediate availability of a newly created RAID logical unit.
It is another object of the present invention to provide methods and associated structure that permit immediate availability of a newly created RAID logical unit prior to initialization of all redundancy information in the logical unit.
It is a further object of the present invention to provide methods and associated structure that permit completion of initialization of a newly created LUN despite failure of a disk drive in the LUN.
It is still a further object of the present invention to provide methods and associated structures to permit interruption of the initialization of a newly created LUN such that the initialization is completed when a failed disk drive is replaced.
It is still another object of the present invention to provide methods and associated structure that permits host I/O requests to be processed in parallel with initialization of redundancy information in a newly defined logical unit.
It is yet another aspect of the present invention to provide methods and associated structure that permit completion of redundancy information initialization in a newly defined logical unit by use of standard RAID regeneration techniques in accordance with the RAID management level of the logical unit.
It is still another aspect of the present invention to provide methods and associated structure to permit interruption of the initialization procedure for a newly defined LUN such that the initialization may be resumed at a later time.